1. Field of the Invention
The present invention relates to a material product comprising a layer of CVD diamond coating applied to a composite substrate of ceramic material and a carbide-forming material of various configurations and for a variety of applications, and methods for manufacturing these products. The products of the invention have utility in a wide variety of applications, which include: heads or disks for the conditioning of polishing pads, including pads used in Chemical-Mechanical-Planarization (CMP), cutting and dressing tool inserts and tips, wear components, such as mechanical seals and pump seals, and heat spreaders for electronic devices.
2. Description of Related Art
CMP is an important process in the fabrication of integrated circuits, disk drive heads, nano-fabricated components, and the like. For example, in patterning semiconductor wafers, advanced small dimension patterning techniques require an absolutely flat surface. After the wafer has been sawn from a crystal ingot, and irregularities and saw damage removed by rough polishing, CMP is used as a final polishing step to remove high points on the wafer surface and provide an absolutely flat surface. During the CMP process, the wafer will be mounted in a rotating holder or chuck, and lowered onto a pad surface rotating in the opposite direction. When a slurry abrasive process is used, the pad is generally a cast and sliced polyurethane material, or a urethane coated felt. A slurry of abrasive particles suspended in a mild etchant is placed on the polishing pad. The process removes material from high points, both by mechanical abrasion and by chemical conversion of material to, e.g., an oxide, which is then removed by mechanical abrasion. The result is an extremely flat surface.
In addition, CMP can be used later in the processing of semiconductor wafers, when deposition of additional layers has resulted in an uneven surface. CMP is desirable in that it provides global planarization across the entire wafer, is applicable to all materials on the wafer surface, can be used with multimaterial surfaces, and avoids use of hazardous gases. As an example, CMP can be used to remove metal overfill in damascene inlay processes.
CMP represents a major portion of the production cost for semiconductor wafers. These CMP costs include those associated with polishing pads, polishing slurries, pad conditioning disks and a variety of CMP parts that become worn during the planarizing and polishing operations. The total cost for the polishing pad, the downtime to replace the pad and the cost of the test wafers to recalibrate the pad for a single wafer polishing run can be quite high. In many complex integrated circuit devices, up to five CMP runs are required for each finished wafer, which further increase the total manufacturing costs for such wafers.
With polishing pads designed for use with abrasive slurries, the greatest amount of wear on the polishing pads is the result of polishing pad conditioning that is necessary to place the pad into a suitable condition for these wafer planarization and polishing operations. A typical polishing pad comprises closed-cell polyurethane foam approximately 1/16 inch thick. During pad conditioning, the pads are subjected to mechanical abrasion to physically cut through the cellular layers of the surface of the pad. The exposed surface of the pad contains open cells, which trap abrasive slurry consisting of the spent polishing slurry and material removed from the wafer. In each subsequent pad-conditioning step, the ideal conditioning head removes only the outer layer of cells containing the embedded materials without removing any of the layers below the outer layer. Such an ideal conditioning head would achieve a 100% removal rate with the lowest possible removal of layers on the polishing pad, i.e., lowest possible pad wear rate. It is apparent that a 100% removal rate can be achieved if there were no concern for its adverse affect on wear of the pad.
However, such over-texturing of the pad results in a shortening of the pad life. On the other hand, under-texturing results in insufficient material removal rate during the CMP step and lack of wafer uniformity. Using the conventional conditioning heads that achieve satisfactory removal rates, numbers of wafer polishing runs as few as 200 to 300 and as many as several thousand (depending on the specific run conditions) can be made before the pad becomes ineffective and must be replaced. Replacement typically occurs after the pad is reduced approximately to half of its original thickness.
As a result, there is a great need for a conditioning head that achieves close to an ideal balance between high wafer removal rates and low pad wear rate so that the effective life of the polishing pad can be significantly increased without sacrificing the quality of the conditioning.
One alternative to the urethane polishing pads described above is a woven or non-woven fiber CMP pad, which may incorporate polyurethane. Like the polyurethane pads, the woven pads are designed for use with an abrasive slurry, but provide an alternative to polyurethane CMP pads that gives finer polishing. While the weave of these pads is quite dense, there is opportunity for slurry particles to become trapped within the weave. These particles must be removed from the weave during conditioning. Efficiency of removal of used slurry components must be balanced against damage to the fibers of the weave caused by contact with the conditioning head surface, which can cause excessive breakage of the fibers.
An alternative to CMP polishing pads designed for use with an abrasive slurry, known as a “fixed abrasive” polishing pad, has been developed in order to avoid the disadvantages associated with using a separate slurry composition. One example of such a polishing pad is the 3M Slurry-Free CMP Pad #M3100. This pad contains a polymer base upon which have been deposited 0.2-micron cerium oxide abrasive in approximately 40 micron tall×200 micron diameter pedestals. These pads also require conditioning, because the CMP polishing rate obtained when using the pads is highly sensitive to the surface properties of the abrasive. Initial “breaking in” periods for these polishing pads, during which consistent quality polishing is difficult to obtain, tend to be long, and the resulting loss of wafers is an added expense. Proper conditioning of these pads can reduce or eliminate this break-in period, and reduce or avoid the loss of production wafers.
Typical diamond-containing conditioning heads have a metal substrate, e.g., a stainless steel plate, with a non-uniform distribution of diamond grit over the surface of the plate and a wet chemical plated over-coat of nickel to cover the plate and the grit. The use of such conventional conditioning heads is limited to the conditioning of polishing pads that have been used during oxide CMP wafer processing, i.e. when the exposed outer layer of the polishing pad is an oxide-containing material as opposed to metal. In processing a semiconductor wafer, there are about the same number of oxide and metal CMP processing steps. However, the conditioning heads described above are ineffective for conditioning polishing pads used in metal CMP processing, because the slurry used to remove metal from the wafer can react with the nickel and degrade and otherwise dissolve the nickel outer layer of the conditioning head. Dissolution of the nickel overcoat can result in a major loss of the diamond grit from the plate, potentially scratching the wafers.
In addition, these typical conditioning heads use relatively large sized diamond grit particles. Similar large particles are disclosed in Zimmer et. al. (U.S. Pat. Nos. 5,921,856 and 6,054,183, the entire contents of each of which are incorporated herein by reference). Instead of using a nickel overcoat, Zimmer et al. bond the diamond grit to the substrate with a chemical vapor deposited polycrystalline diamond film (“CVD diamond”). The diamond grit, commercially available from the cutting of natural diamonds and front industrial grade diamonds using high-pressure processes, is incorporated into the structure of the thin CVD diamond film. The size of the grit is chosen so that the peak-to-valley surface distance is grater than the thickness of the CVD diamond film. The diamond grit is uniformly distributed over the surface of the substrate at a density such that the individual grains are separated by no less than ½ the average grain diameter. The average size of the diamond grit is in the range of about 15 microns to about 150 microns, preferably in the range of about 35 microns to about 70 microns. By controlling the size and surface density of the diamond grit, the abrasive characteristics of the resulting surface can be adjusted for various conditioning applications. The grain sizes on a given disk will be relatively consistent in size, to approximately ±20%.
Roughness of a surface can be measured in a number of different ways, including peak-to-valley roughness, average roughness, and RMS roughness. Peak-to-valley roughness (Rt) is a measure of the difference in height between the highest point and lowest point of a surface. Average roughness (Ra) is a measure of the relative degree of coarse, ragged, pointed, or bristle-like projections on a surface, and is defined as the average of the absolute values of the differences between the peaks and their mean line. RMS roughness (Rq) is a root mean square average of the distances between the peaks and valleys. “Rp” is the height of the highest peak above the centerline in the Sample length. “Rpm” is the mean of all of the Rp values over all of the sample lengths. Rpm is the most meaningful measure of roughness for gritless CMP pads, since it provides an average of the peaks that are doing the bulk of the work during conditioning. However, a new generation CMP pads, including fixed abrasive pads and many woven pads, cannot be conditioned by conventional conditioners because conditioning heads having grit particles larger than 15 microns are too rough; the large grit particles tend to damage the pad.
An alternative to using diamond grit is disclosed in U.S. Ser. No. 10/091,105, filed Mar. 4, 2002, the entire contents of which are incorporated herein by reference. The application describes the use of CVD diamond coating on a polished silicon substrate, without the use of diamond grit, to create the abrasive surface and to control the conditioning rate. The surface roughness resulting from simply growing CVD diamond on a silicon substrate ranges from about 6 to 12 microns from peak-to-valley on a substrate having a thickness of 25 microns of CVD diamond. In general, the surface roughness for a typical operation ranges from about ¼ to about ½ the thickness of the CVD diamond that is grown on the substrate. This degree of surface roughness can provide the desired abrasive efficiency for CMP conditioning operations for fixed abrasive CMP pads. However, difficulties with this approach are the lack of independent control of the particle size and density of working diamond grains, and the resulting bow of the diamond-coated silicon substrate product.
While silicon has been used successfully as a substrate for CVD diamond in preparation of some CMP pad conditioners, in accordance with the invention of this application, it has been found that a silicon substrate does not provide sufficient rigidity to support diamond coatings of sufficient thickness to provide optimal CMP conditioning in some applications with sensitive pad materials. Because of both internal growth stress in CVD diamond materials, and the mismatch in thermal coefficients of expansion between diamond and silicon, a CVD diamond-coated silicon substrate conditioning head will bow or bend, even when supported by a metal backing plate, resulting in a conditioner that is not completely flat. A bowed conditioning head does not provide as consistent conditioning as a flat conditioning head, and is thus less desirable.
In addition to CMP pad conditioners, CVD diamond materials and coatings are also used in applications such as cutting tools, wear components and heat spreaders. In each of these applications, the selection of the substrate on to which the diamond is deposited is governed by a number of considerations, including not only the mechanical, thermal, chemical, and/or electrical properties necessary for that particular application, but also by the ability of the CVD diamond to adhere to the substrate. The deposition of polycrystalline CVD diamond onto a number of different substrates has been reported. These substrates can include metals, such as tungsten, molybdenum, tantalum, silicon, copper, aluminum, or non-metals, such as carbon, silica glass, alumina, silicon carbide, tungsten carbide, titanium carbide, silicon nitride, and boron nitride.
The most commonly used material for cutting tools is “high speed” steel. However, this material is not well suited as a substrate for growing CVD diamond because the deposition process reduces the hardness and strength of the steel. Cemented tungsten carbide is also used frequently in making cutting tools, but CVD diamond does not adhere well to this material without either etching of the surface to remove cobalt, or application of an intermediate layer of material to increase adhesion. Likewise, silicon nitride, another material used in the cutting tool industry, does not provide the high adhesion necessary for CVD diamond coatings for viable commercial application. Use of molybdenum as a substrate results in a high CVD diamond nucleation density, but again the adhesion of CVD diamond to this substrate is poor. CVD diamond does adhere well to silicon substrates, but silicon is too fragile and has a fracture toughness that is too low, to make it suitable as a substrate for cutting tools, wear parts or other mechanical applications.
The use of CVD diamond as a heat spreader for high power density electronic components such as laser diodes, LED's, RF components, and other passive electronic components such as resistors allows reduced junction temperature and increased mean time between failures, or much higher power densities to be achieved with these components. In many applications, however, the use of freestanding CVD diamond substrates is inhibited either by the size of the die, the thickness of the die needed for retrofit into existing packages, and/or the cost of the diamond material for the given application. Whether the cost-to-benefit ratio justifies a freestanding CVD diamond heat spreader depends at least in part on the power densities generated by a particular device. Thus, there still exists a need for a low-cost heat spreader incorporating CVD diamond as an alternative to freestanding CVD diamond heat spreaders.
CVD diamond coatings have been proposed previously as a method of improving wear resistance of a variety of materials, including silicon carbide. The high shear forces and temperatures that are generated in, for example, a dry-running pump seal application, generally result in adhesion failure of CVD diamond coatings on typical pump seal materials, such as sintered silicon carbide. There is still an unmet need for a process to manufacture CVD diamond-coated pump seals that can withstand dry running conditions.
In summary, there remains a need in the art for polishing pad conditioning materials that do not damage CMP pads, and that provide improved surface roughness characteristics, and that do not suffer from bowing. In addition, there remains a need for a substrate material that provides desirable dimensional stability, thermal characteristics, chemical and abrasion resistance, and adhesion to CVD diamond, suitable for use in preparing polishing pad conditioners, cutting tools, wear parts such as pump seals, and heat spreading substrates for electronics.